Zener diodes are commonly used as voltage clamps to provide voltage overload protection. As the magnitude of the voltage and current overload levels to be controlled increases, the size and cost of a zener diode increases commensurately. The cost of zener diodes to afford overload protection from voltage surges of hundreds of volts and tens of amperes usually makes such zener diodes economically nonviable.
For high voltage operation, TRIACs are commonly used. TRIAC voltage overload protectors require a high holding current which is difficult to achieve. In addition, TRIACs are sensitive to dv/dt changes and can be turned on by a relatively small voltage change which occurs in a very short period of time. It is desired that a protection device only turn on if the magnitude of the voltage overload reaches a preselected level.
It is desirable to have solid-state protector circuitry which can relatively rapidly discharge voltage surges of hundreds of volts and at least several amperes when the magnitude of the voltage overload reaches a selected potential and which is relatively insensitive to dv/dt changes.
There have recently become available new forms of solid-state switches which can be operated at high voltages and at high currents. Switches of this kind are disclosed in copending U.S. patent applications Ser. Nos. 972,056 (A. R. Hartman-T. J. Riley-P. W. Shackle), 971,886 (A. R. Hartman-B. T. Murphy-T. J. Riley-P. W. Shackle), and 972,021 (A. R. Hartman-A. U. MacRae-P. W. Shackle), filed concurrently with this application, and in which there are some common inventors and a common assignee, and in the article entitled "A Field Terminated Diode" by Douglas E. Houston et al., published in IEEE Transactions on Electron Devices, Vol. ED-23, No. 8, August 1976.